Relative image files not found. Background render. No error, just magenta textures
Got it.  The clue was that another asset, a movie, was found and rendered, but it was in a subdir instead of being the the same dir as the blend file.  So, I moved the images from the same dir as the blend file to their own images subdir.  Then, open the blend file and select File > External Data > Make all files relative.  Save.